Method for peroxide bleaching of pulp

ABSTRACT

Bleaching of ligneous cellulosic materials such as paper pulp by treating the pulp with a combination of tertiary butyl hydroperoxide and hydrogen peroxide as the bleaching agent.

United States Patent Lincoln et al.

METHOD FOR PEROXIDE BLEACHING OF PULP Inventors: Robert M. Lincoln,Moylan; Joseph A.

Meyers, Ill, Springfield, both of Pa.

Assignee: Atlantic Richfield Company, New York, N.Y.

Filed: Jan. 19, 1971 Appl.No.: 107,843

Related U.S. Application Data Continuation-impart of Ser. No. 781,989,Dec. 6, 1968, abandoned.

[ 1 Feb. 29, 1972 Primary Examiner-Howard R. Caine Attorney-John J.McConnack and John D. Peterson ABSTRACT Bleaching of ligneous cellulosicmaterials such as paper pulp by treating the pulp with a combination oftertiary butyl hydroperoxide and hydrogen peroxide as the bleachingagent,

4 Claims, No Drawings 1 METHOD FOR PEROXIDE BLEACHING OF PULP CROSSREFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our copending application Ser. No. 781,989,filed Dec. 6, 1968, entitled METHOD FOR PEROXIDE BLEACHING OF PULP, andnow abandoned.

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This inventionrelates to a method for producing ligneous cellulosic materials ofimproved brightness, in particular paper pulp by bleaching with tertiarybutyl hydroperoxide in combination with hydrogen peroxide.

2. PRIOR ART Hydrogen peroxide has been used advantageously in bleachinghigh yield pulps such as groundwood, despite its high cost since withcareful use it can bleach without causing yield loss or without reducingthe degree of polymerization of the cellulose. Hydrogen peroxide alsohas been used as a finishing bleach in draft paper manufacture after thenoncellulosic components have been reduced by a series of stepsincluding chlorination, alkaline washing, and chlorine dioxidebleaching.

The chemistry of hydrogen peroxide bleaching is incompletely understood,however, it is believed that the hydrogen peroxide in solution ionizesreversibly to form a hydrogen ion and a perhydroxyl ion, the latter isbelieved to be the active species in the bleaching reaction. Hydrogenperoxide can also decompose to release free oxygen. The oxygen releasedby this decomposition of hydrogen peroxide is said not to have anybleaching action but instead may actually be harmful by causing chainscission of the cellulosic molecules. Therefore, care must be exercisedto prevent this reaction.

This hydrogen peroxide decomposition reaction occurs even at roomtemperatures and is rapid at higher temperatures or in the presence ofcatalytic materials such as metal ions.

In our copending application, Ser. No. 753,713, filed Aug. 19, 1968,entitled METHOD FOR BLEACHING PULP it was disclosed that an organichydroperoxide such as tertiary butyl hydroperoxide, in particular, canbe utilized for bleaching paper pulp instead of hydrogen peroxide. Itwas shown that the paper pulp can be bleached by replacing hydrogenperoxide with an equal amount of the tertiary butyl hydroperoxide andthat the brightness increase obtained with the hydroperoxide was foundto be the same as for the hydrogen peroxide. The advantage of using thetertiary butyl hydrogen peroxide is that such compound is quite stableand, in general, decomposes only at temperatures above about l-1 C. inthe absence of reducing agents.

Various peracids in particular peracetic acid in conjunction withchlorine dioxide have also been proposed as bleaching agents. Peraceticacid, however, is extremely unstable and decomposes with explosiveviolence at about 110 C. and therefore must be handled with extremecaution even at low temperatures.

That tertiary butyl hydroperoxide can be utilized as a bleaching agentis completely unexpected in view of the wide differences in chemicalreactivity among tertiary butyl hydroperoxide, hydrogen peroxide and theperacids. For example, olefins, in general, are converted smoothly andin good yields to the corresponding epoxides by peracids at lowtemperatures (O25 C.) while similar results are obtained withhydroperoxides only at higher temperatures (90-l20 C.) and only in thepresence of a transition metal catalyst such as molybdenum. Hydrogenperoxide does not epoxidize olefins either in the presence or in theabsence of catalysts.

Ketones are converted to esters and lactones by peracids while muchpoorer results are obtained with hydrogen peroxide and then only in thepresence of a base as a catalyst. I-Iydroperoxides on the other band donot react with ketones.

Base catalyzed hydrogen peroxide converts aromatic aldehydes and ketoneshaving an amino or a hydroxyl group in the ortho or para position to thecorresponding amino phenols or polyphenols but neither hydroperoxidesnor peracids can be used in this reaction. These differences in chemicalreactivity therefore render the use of tertiary butyl hydroperoxide as ableaching agent to be completely unexpected.

Tertiary butyl hydroperoxide provides additional advantages overhydrogen peroxide and the peracids. It is stable in aqueous solutionsand does not require stabilizing agents. It does not lose its bleachingstrength in the presence of metallic ions such as Fe, Co, Ni, etc., ionswhereas hydrogen peroxide for example must be stabilized and is mosteffective only with sequestering agents being present or alternativelyby removing the metal ions from the pulp solution prior to bleaching.

Tertiary butyl hydroperoxide has an exceedingly important advantage overhydrogen peroxide or the peracids such as peracetic acid in that it isan excellent agent for preventing microbiological attack on the pulp.This feature is a unique and important advantage since molds or othermicrobiological growths on pulp follow through the process and finallydegrade the quality of the finished paper.

The method of the present invention employs a combination of hydrogenperoxide and tertiary butyl hydroperoxide as the bleaching agents. Whilethis method does not completely obviate the disadvantages of usinghydrogen peroxide alone it does provide the advantages of using tertiarybutyl hydroperoxide, i.e., economy and inhibition of microbiologicalgrowths. It also provides an additional advantage, namely, that anonpolluting environment can be employed, i.e., the alkaline agentemployed in conjunction with the bleaching agents may be sodiumcarbonate or sodium bicarbonate instead of the sodium silicateheretofore required with the hydrogen peroxide system. Sodium silicatesare polluting agents and thus generally found their way into the wastewater stream or otherwise required costly recovery methods.

The present method also provides an additional advantage, namely, it canbe carried out at ambient temperatures. Heretofore, the hydrogenperoxide system had to be carried out at high temperatures for shorttimes in order to minimize mold growths. The present method allowsambient temperature treatment giving the desired brightness improvementwhile completely inhibiting mold growth. Ambient temperature treatmentis particularly desirable with hydrogen peroxide present since thislessens the attack on the cellulose fibers and provides a superior pulpfor paper manufacture.

SUMMARY OF THE INVENTION In accordance with this invention a combinationof tertiary butyl hydroperoxide with hydrogen peroxide is utilized tobleach ligneous cellulosic materials such as pulp for paper manufacture.

It is an object of this invention therefore to provide an improvedmethod for bleaching ligneous cellulosic materials.

It is another object of this invention to provide a method for bleachingligneous cellulosic materials utilizing the combination of tertiarybutyl hydroperoxide with hydrogen peroxide as the bleaching agent.

Other objects of this invention will be apparent from the description ofthe preferred embodiments which follows and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of this inventionis particularly adapted for the bleaching of cellulosic fibers andmaterials that are derived from lignin-containing materials by any ofthe processes that are generally used for fiber liberation.

The invention is particularly applicable to the bleaching of groundwoodpulp, i.e., pulp containing large amounts of lignin. Heretoforeapproximately percent of the hydrogen peroxide utilized in pulpbleaching has been for the bleaching of groundwood pulp, however, thisinvention is also applicable to treating any pulps which may be treatedwith hydrogen peroxide at some stage in their manufacture, for example,kraft pulp.

In recent years a method for the epoxidation of olefinic compounds hasbeen developed utilizing an organic hydroperoxide as the oxidizing agentin a catalytic process. The large requirement for hydroperoxides forthis process has led to the development of a convenient and economicprocess for producing organic hydroperoxides in particular tertiarybutyl hydroperoxide by the noncatalytic oxidation of isobutane withmolecular oxygen. This process has been known for a number of years andis now being developed commercially. Consequently, tertiary butylhydroperoxide can be produced at a price much lower than that ofhydrogen peroxide.

Since it has been found in accordance with this invention that tertiarybutyl hydroperoxide can be combined with hydrogen peroxide to givebrightness values greater than with hydrogen peroxide alone, this methodhas the additional advantage of permitting the use of tertiary butylhydroperoxide in place of simply larger quantities of hydrogen peroxide.

The use of tertiary butyl hydroperoxide in combination with hydrogenperoxide therefore extends the range of brightness possible by a givendosage of hydrogen peroxide, and makes exact dosage of the hydrogenperoxide unnecessary. Exact dosage is normally required to preventcellulose damage, but excess hydroperoxide can be used and recovered.

Although it is extremely difiicult to demonstrate quantitatively, it isbelieved that another very important advantage of the hydrogenperoxide-tertiary butyl hydroperoxide combination of this invention liesin the milder action of the combination. Hydrogen peroxide alone israpid, severe and indiscriminate in its attack, particularly since it isused at elevated temperatures to minimize mold growth, thus by limitingthe amount of hydrogen peroxide employed and by allowing lower treatingtemperatures there are two realizable advantages one in prevention ofyield loss and the second in improvement in pulp quality.

Hydrogen peroxide treatment is followed by a sulfite aftertreatment inorder to avoid color reversion due to active oxygen-containing compoundsremaining in the pulp and to complete the decolorization of the colorbodies to colorless compounds. Sodium hydrogen sulfite (sodiumbisulfite) can be used as an after-treatment compound although othersulfites such as sulfur dioxide bubbled into the system also can beutilized. With the hydrogen peroxide-tertiary butyl hydroperoxidetreatment, sulfite treatment can be omitted.

The amount of hydrogen peroxide utilized to treat the pulp is in thesame range as that utilized by the industry when hydrogen peroxide isused as the sole bleaching agent. In general, amounts ranging between0.1 weight percent to 5 weight percent of the dry pulp can be usedalthough even larger amounts can be employed. Amounts in the range offrom 0.5 to 2.0 are generally satisfactory when used in combination withthe tertiary butyl hydroperoxide. The amount of tertiary butylhydroperoxide may range between about 2 weight percent to weight percentbased on the weight of the dry pulp, with about 5 weight percent beinggenerally satisfactory.

The amount of sulfite, for example, sodium hydrogen sulfite for theafter-treatment can range between 1 weight percent and 6 weight percent,although this is not critical, and higher or lower amounts may beemployed. Preferably an aqueous solution of about 10 percentconcentration is employed and the amount used is about 10 percentconcentration is employed and the amount used is sufficient to adjustthe pll to the desired level, for example, a pl-i of 6.8 although thismay be varied as experience dictates.

Sodium silicate is the recommended butter for hydrogen peroxidebleaching according to the prior art and it is known that hydrogenperoxide is less effective in the absence of a buffer. It has also beenfound in the instant invention that it is preferable to employ a bufferin the bleaching step. Sodium silicate also can be employed in thisinvention, i.e., the ordinary commercial material known as water glass.in general, the commercial sodium silicate wherein the Si0 to Na O ratiois from 1.6:1.0 to 2.4:].0 is satisfactory and the amount of sodiumsilicate can vary between about 3 weight percent and 7 weight percentbased on the weight of the dry pulp with 5 weight percent being quitesuitable. Since silicates present pollution problems it is preferred inthis invention to employ either sodium carbonate or sodium bicarbonateas the buffer since neither of these present any pollution problems.Between 3 and 7 weight percent are also suitable for these butfers.

It has also been found preferable to include a small quantity of sodiumhydroxide. In general, the amount of sodium hydroxide is approximatelyone-half the weight of the buffer, i.e., in general, about 2.5 weightpercent being satisfactory, although amounts ranging between 1 percentand 4 percent can be used. Sodium hydroxide can be completelyeliminated, however, without loss in brightening improvement.

In order to demonstrate bleaching effectiveness of the combination ofhydrogen peroxide with tertiary butyl hydroperoxide a number of runswere carried out to compare hydrogen peroxide alone with the combinationof hydrogen peroxide and tertiary butyl hydroperoxide.

Laboratory tests were standardized and based on TAPPl Routine Controltest methods. The pulp employed was a commercial groundwood. The pulp asreceived has a Photovolt Reflectance (brightness) of 70. It is wellrecognized in this art that an acceptable color improvement by hydrogenperoxide or similar competitive bleaching agents is 10 units, i.e., aPhotovolt value of 80.

The test method consists of first conditioning the pulp. The pulp asreceived contains approximately 83 percent water. A slurry of this pulpin water was made by admixing 1 weight of pulp as received with 2weights of water. This mixture was allowed to stand overnight and thenmixed for 10 minutes with a wire beater and filtered in a Buchner funnelfor 15 minutes using a rubber dam. The resulting cake is used as thestock for the bleaching test. The cake in general contains about 20percent dry pulp. A sufficient amount of the stock prepared as describedabove contained in a polyethylene bag is admixed with aqueous hydrogenperoxide, aqueous sodium silicate or other buffer, aqueous sodiumhydroxide (if used), an aqueous solution of tertiary butyl hydroperoxideand sufficient additional water to give a 10 percent consistency, i.e.,10 grams of dry pulp and grams of total water. The hydrogen peroxide,buffer and tertiary butyl hydroperoxide were in amounts sufficient togive the weight concentrations based on the dry pulp as shown in thetable in the Example. In each case the water is adsorbed by the pulp andthere is little free liquid.

The polyethylene bag is kneaded to mix the pulp, bleaching solution andbuffer and is then immersed in a water bath at 170 F. for one hour. Onremoval from the bath the aftertreating agent is mixed by kneading andafter 15 minutes the pulp is poured onto a Buchner funnel where thesuperficial liquid is removed through a filter paper. The after-treatingagent in the runs in the Example was a 10 percent aqueous sodiumhydrogen sulfite solution in an amount sufficient to adjust the pH to avalue of 6.8.

After removal of the superficial liquid the pulp on the funnel iscovered by a second piece of filter paper and placed between layers offelt. The pulp bed covered by the filter paper and felt is squeezedbetween rubber rollers and then dried in an oven at 110 C. for one hourwith the filter paper covers in place. Following the oven drying stepthe amount of bleaching obtained is detennined by measuring thereflectance by Model 610 Photovolt Reflectometer and comparing it with ahand sheet made by the same procedure without bleaching orafter-treating solution.

The Photovolt Reflectometer for this purpose uses a blue filter and thebrightness values thus obtained can be converted to TAPPl officialGeneral Electric brightness values. The Photovolt Reflectometer isadjusted so that a standard magnesium oxide block reads percentreflectance.

In the runs which follow no values are shown for treating with tertiarybutyl hydroperoxide alone, since it was found that under these testconditions that a brightness improvement of from only two to four pointswas obtained.

The runs in the following examples are provided to demonstrate theeffectiveness of the combined hydrogen peroxide EXAMPLE 1 A series ofruns were made on Poplar (hardwood) groundwood utilizing the procedureset forth above. The results of 7 these tests are set forth in the Tabletogether with the test conditions for each run. The amounts set forthare weight percent based on the weight of dry pulp.

TABLE Percent Percent Percent sodium Percent Bright- H O BHP silicateNaOH ness These results show that the same brightness improvement can beobtained by using only 0.25 weight percent H O and.5 percent tertiarybutyl hydroperoxide as can be obtained with twice the amount of H 0alone. Similarly, at a brightness level of 87, 1 weight percent H O with5 weight percent tertiary butyl hydroperoxide can be used and isequivalent to twice the weight percent H 0 i.e., 2 weight percent. At abrightness level of 89 which is 19 points above the starting level of 70for the unbleached pulp, a 50 percent increase in H 0 is required to beequivalent to 2 weight percent H O together with 10 weight percenttertiary butyl hydroperoxide. Accordingly, these results show theusefulness of the method of this invention.

The following Examples are provided for the additional purpose ofdemonstrating the ability of tertiary butyl hydroperoxide to improve thehydrogen peroxide bleaching of groundwood (mechanical) pulps by (a)replacing the prior art polluting alkali silicate with nonpollutingalkali carbonate or bicarbonate and (b) allowing storage-bleaching atambient temperatures with the hydrogen peroxide-tertiary butylhydroperoxide agents, wherein the tertiary butyl hydroperoxide alsoprevents mold formation.

EXAMPLE ll Into a polyethylene bag was weighed 18.8 grams (3.0 grams,oven dried) of white spruce refiners groundwood, 1.0 grams of 10 weightpercent hydrogen peroxide, (3.3 weight percent of dry pulp), 5.0 gramsof 10 percent sodium carbonate (aq.) and 15.0 grams of distilled water.The pulp was manually mixed thoroughly with the aqueous additives togive a pulp of 7.5 percent consistency which showed no free liquid butfrom which liquid could be easily squeezed.

In a parallel run the above conditions were repeated and in addition,0.3 gram of tertiary butyl hydroperoxide was included. After standing atroom temperature for 10 days, the pulp without tertiary butylhydroperoxide had darkened, as compared to untreated pulp. The darkeningwas due to mold formation which was seen in discrete colonies which in 3weeks had permeated the entire pulp.

The bag containing tertiary butyl hydroperoxide after 5 days at roomtemperature was free of mold and had brightened as shown by Photovoltreadings on handsheets from the untreated reading of 75 to 83.

Observation of the pulps after 2 months showed greatly increasing moldformation and darkening in the pulp containing no tertiary butylhydroperoxide, the tertiary butyl hydroperoxide treated sample was moldfree and had brightened considerably over the handsheet which read 83after 5 days.

EXAMPLE Ill The procedure of Example 11 was repeated using 0.3 gram ofsodium bicarbonate in place of the 0.15 gram of sodium carbonate. Asbefore, parallel runs were made (a) without tertiary butyl hydroperoxideand (b) with 0.3 gram tertiary butyl hydroperoxide. The results after 5days at room temperature were, to the eye, the same as the sodiumcarbonate pulps of Example II. A Photovolt reading on a handsheet after5 days read 82 for the tertiary butyl hydroperoxide treated pulp. Thepulp with no tertiary butyl hydroperoxide added was darker, after 5 daysat room temperature, than the starting pulp and mold colonies wereeasily seen. After 2 months the tertiary butyl hydroperoxide treatedsample had continued to brighten while the pulp with no tertiary butylhydroperoxide present was uniformly moldy. Little difference betweensodium carbonate and sodium bicarbonate is seen with respect to moldformation.

EXAMPLE IV The procedure of Examples ll and 111 was repeated using 0.30gram of 40-42 Baume sodium silicate in place of sodium carbonate orbicarbonate.

The results differed from those of Example 11 and Example III, in that,silicate and hydrogen peroxide gave brightening at room temperature inthe absence of tertiary butyl hydroperoxide. However, the brightnessincrease was negated by formation of mold colonies, so that, after 5days some areas of the pulp were as bright as the tertiary butylhydroperoxide treated pulp, but, overall, the mold colonies caused anoverall brightness inferior to the tertiary butyl hydroperoxide treatedsample. After 2 months, mold formation was extensive in thesilicate-hydrogen peroxide treated pulp while the tertiary butylhydroperoxide (0.3 gram) containing pulp was mold free.

Photovolt readings on the tertiary butyl hydroperoxide treated hydrogenperoxide, silicate bleached pulp after 5 days were 85. This compareswith Photovolt readings of 75 for the unbleached white sprucegroundwood; 82 for hydrogen peroxide, sodium bicarbonate, tertiary butylhydroperoxide treated pulp and 83 for hydrogen peroxide, sodiumcarbonate, tertiary butyl hydroperoxide, treated pulp.

The brightness obtainable by the methods of Examples ll and 111 can beincreased by the usual above-discussed sulfite after-treatment. Sodiumhydroxide, however, is not employed with the sodium carbonate orbicarbonate.

The importance of the foregoing Examples is emphasized by reference toTAPPI Monograph No. 27 Bleaching of Pulp 1963, wherein on page it isstated that mechanical pulps are preferably bleached with hydrogenperoxide immediately after grinding, however when mold and spoilage ofpulp is absent storage for 6 months or more is practical. On page 191 ofthe monograph it is pointed out that certain bacteria inherently presentin the wood, reduce hydrogen peroxide efficiency by producing catalase"which greatly accelerates hydrogen peroxide decomposition. l-leretoforethe bacteria were controlled by heat, chlorinating agents andbactericides usually heavy metal compounds such as mercurial compoundsall of which are polluting materials.

In the instant process any unreduced tertiary butyl hydroperoxide isrecovered by separation of its reduction product, tertiary butylalcohol. The tertiary butyl alcohol is easily separated from the waterby distillation in the form of its azeotrope with water, and is readilyburned in this form to produce carbon dioxide and water which step alsoproduces useful process heat. Thus, the instant method is completelynonpolluting either with respect to air pollution or with respect towater pollution.

We claim:

1. A nonpolluting method of bleaching ligneous cellulosic fibers whichcomprises contacting the cellulosic fibers with an aqueous solution ofhydrogen peroxide and tertiary butyl hydroperoxide in the presence of abuffer, said hydrogen peroxide ranging in an amount of from about 0.1weight percent to 5.0 weight percent based on the weight of the drycellulosic fibers, said hydroperoxide ranging in an amount from 2 weightpercent to weight percent based on the weight of the dry cellulosicfibers and said buffering agent being sodium carbonate or sodiumbicarbonate and ranging from 3 weight percent to 7 weight percent basedon the weight of the dry cellulosic fibers.

2. The method according to claim 1, wherein the cellulosic fibers whichhave been treated with hydrogen peroxide, tertiary butyl hydroperoxideand buffer are treated with a sulfite compound said sulfite compoundranging in an amount from about 1 weight percent to 6 weight percentbased on the weight of the dry cellulosic fibers.

3. The method according to claim 1, wherein the said buffer is sodiumcarbonate.

4. The method according to claim 1, wherein said buffer is sodiumbicarbonate.

2. The method according to claim 1, wherein the cellulosic fibers which have been treated with hydrogen peroxide, tertiary butyl hydroperoxide and buffer are treated with a sulfite compound said sulfite compound ranging in an amount from about 1 weight percent to 6 weight percent based on the weight of the dry cellulosic fibers.
 3. The method according to claim 1, wherein the said buffer is sodium carbonate.
 4. The method according to claim 1, wherein said buffer is sodium bicarbonate. 